Protocols for sharing a wireless medium effectively among multiple users are generally denoted multiple access protocols, channel access schemes or medium access schemes. Multiple access protocols may be divided into two main categories: conflict-free protocols and contention-based protocols.
Conflict-free protocols ensure that a transmission, whenever made, is successful, i.e. not interfered by other transmissions. Conflict-free transmission can be achieved by allocating the channel to users either statically or dynamically. This is often denoted fixed and dynamic scheduling, respectively. The benefit of the precise coordination among stations is that it is believed to provide high efficiency, but this comes at the expense of complexity and exchange of sometimes large quantities of control traffic.
Contention-based protocols differ in principle from conflict-free protocols in that transmissions have a tendency to be less successful. The protocol should therefore prescribe procedures to resolve conflicts or unsuccessfully transmitted data once they occur so that all messages are eventually transmitted successfully.
A typical example of such a conflict-resolving procedure is requested retransmissions in the form of Automatic Repeat Request (ARQ). In ARQ systems, the receiving node detects whether a received data block or packet includes errors, i.e. whether it can be successfully decoded or not. When errors are detected, a retransmission request, in the form of a Negative Acknowledgement (NACK) identifier, is forwarded to the transmitting node. On the other hand, in response to a successful, i.e. error free, packet reception and decoding, an Acknowledgement (ACK) identifier is returned. As a consequence, error free conditions can be achieved, though, at the cost of long transmission delay times due to the, sometimes many, retransmissions.
An improvement of the (simple) ARQ scheme is the repeated data packet transmission in the form of Hybrid ARQ (HARQ). The HARQ schemes can use the information from a previously erroneous or unsuccessful transmission in order to improve the chances of decoding a following data (re)transmission.
As is known in the art, if a receiving side cannot decode the original data packet correctly, a NACK is typically returned to the transmitting side. If possible, soft values of the original data packet may be stored in a memory at the receiving side. In response to the NACK, the transmitting side will retransmit the data packet (in Chase Combining (CC) HARQ schemes) or transmit incremental redundancy data (in Incremental Redundancy (IR) HARQ schemes). The receiving side can then utilize soft values of the original data block and combine them with soft values of the presently received data packet in order to increase the chances of successful decoding.
In an extension of HARQ, denoted Reliability-Based HARQ (RB-HARQ) [1, 2], the simple NACK is exchanged by information identifying the weakest (most unsuccessfully decoded) bits in the original data packet. As a consequence, the retransmitted data packet only has to include these weakest bits, resulting in smaller retransmission sizes. In a patent application [3], an adaptive HARQ scheme is employed, where the amount of redundancy to transmit in response to a notification of failed decoding is determined based on a quality estimate of an unsuccessfully received data block.
However, although the RB-HARQ schemes [1, 2] and adaptive HARQ scheme [3] reduce the amount of retransmitted data they fail to increase the throughput of wireless communications networks by providing a scheduling of data packets that exploits opportunities, which enable signal transmissions to be more successful than at other times and conditions. Such opportunities will typically arise due to changes or fluctuations within the network over time. An opportune routing [4-6] partly mitigates the rapidly varying link quality in the network by making use of windows of opportunity that these fluctuations provide. Although the existing opportunistic routing schemes [4-6] work well in most situations, they are not particularly suitable for HARQ-based data communication. Thus, there is a general need of providing an efficient opportunistic data packet routing and scheduling scheme adapted for HARQ-based communication.